My temp gauge in my 68 BB 390 did not work once I started the engine after it set for 15 years. I car runs great but I’m struggling to get a working temperature gauge for it. After replacing the sending unit I still had no movement on the gauge, even when I dead shorted the green wire lead while doing a simulated bench test. So I bought a new gauge. It bench checked great! Plugged in the temp gauge pigtail into the new gauge and run the engine till my infra-red gun showed a temperature of 180 at the T-stat and 240 at the exterior of the sending unit. STILL no movement of off 100!@#$??

Summary of Retest:
1. Red wire: 12+ Volts.
2. Black wire: Grounded, in fact it blew the 10 amp gauge fuse when I was careless.
3. Green wire: Continuity only, I did not load check it.
4. I monitored the resistance across the sending unit case and output terminal during engine cool down. The following is a summary of the temperature on the surface of the sending unit and the ohm reading across the gauge:
1. Test Start 240 degrees 103 ohms
2. 25 minutes 135 degrees 130 ohms
3. 40 minutes 125 degrees 163 ohms
4. 1 hr 13 min 101 degrees 195 ohms
5. 2 hr 10 min 96 degrees 1 At initial contact various ohm reading would flash up but then immediately go to 1. Doesn't these results show that the sending unit is good?

Two thoughts, 1) should the gauge unit itself be grounded as if it were setting in the gauge cluster and 2) if my firewall connection was badly corroded could I have continuity but not current flow??
I’m electrically challenged and any and all help will be greatly appreciated. Hugh

 

Ok i looked up the '68 vette wiring diagram. I think the gauge has 3 wires. A green wire that connects to the sensor on the engine. A black wire that just goes to ground (just maybe in a unseen location from the gauge). And a pink wire that brings 12vdc batt power to the gauge.

So if u measure the pink wire on the gauge to ground it should read exactly batt voltage.

So if u measure the black wire on the gauge to ground it should read zero as the same potential as ground anywhere in the car.

Now the green wire will differ in voltage as the temp of sensor/sender changes.

Ok your resistance readings show a operating thermister with a negative temp coefficient. As temp increases the resistance decreases or in your case as temp decreases the resis increases. I cant explain how 96*F is only 1 ohm but the rest look reasonable.

What u want to do now is "back pin" the sensor connector (green wire) and read the DC voltage (red pos meter lead on the sensor and black neg lead to ground) . This should be the same as what u measure on the same green wire on the gauge terminal (same pos meter wire on gauge terminal and neg meter wire to gnd). U can do this just by attaching extension wires.

I think u find large difference between the 2. Next measure continuity (resistance) from the end of the sensor green wire to the gauge terminal connector for the green wire - resistance should be near zero. Im guessing its not and u need to repair the wiring.

Yes the gauge is grounded through that black wire. The sensor is grounded by its case and sometimes sealant or Teflon tape prevents this - just check resistance from the case to the head (should be near zero). No u really cant have continuity and no current flow in an energized circuit. Now a de-energized circuit with one lead lifted will show no resistance right up to the open in the circuit.

Thats enough for now. Hope it helped.

 

Thank you. I should have said that at ambient temperature the sending unit reads that the circuit is 'open'. My ohm meter read 1 if the circuit open and 0 if leads connected to each other. I'm sure they all do that its just that I did not make myself clear. I agree that the problem is probably the green wire. I'll do as you suggested if it stops raining today. I have to run that BB outside. Hugh

 

The ground on the case may be the problem if all the wiring is good. I read that if the console is cracked your ground will be intermittent at best

 

If the gauge has black wire it is a dedicated ground for the gauge.

 

had the same problem. ground was good. followed the Wilcox instructions and determined my resistor was good, but not making contact properly to the studs. it is very easy to over tighten the nuts and scrape off the contact area. my vette came with 4 gauges, all GM, 2 still new in the box...PO had the same problem, but I ended up using new resistor with the correct washers/insulators and just enough torque to make contact with the original '68 guage...

look at the picture on Wilcox's site and you'll see exactly what I am talking about

 

Always let the gauge tell you what the issue is.. No movement off 100...

 

My 68 BB temp gauge would not move off 100. The fiber washer was installed but looked "old" so I replaced it and that solved my problem.

 

Thanks again to all the folks that responded. The Wilcox 68-76 Temp Gauge Assembly Instructions, page 2/17 was of particularly good help. I did not have the back of my gauge exactly like the one Wilcox shows. Mine does not have a hole for the tab on the ground (center) lead to fit into, see photo. Because of that I put insulator on first. I corrected everything to look like Wilcox’s. Now my new gauge is not passing the bench check. Now I have three gauges that bench test with a bad ‘ohm’ wire input (only moves to 100 deg) even though I have it dead shorted.
On my new gauge, I have 87 ohms across the insulator, 48 ohms across the power/ohm wire leads, 158 ohms across the ohm wire/ground leads, an open across the power/ground leads and 0 ohm from the ground lead to the case. I'm at a loss. Can the circuit inside the gauge burned out?? Hugh






Loose lead is the power lead.

 

and you put it together like this... ?? If not double check.. when you bench assemble one for testing you must make sure the terminals are insulated too.. not touching the metal can.

Sure the winding in side can be burnt, I've never ohms tested them because if they are assembled properly and don't work I know they are toast.. but on three gauges, that doesn't make sense unless you have something cooking them.

 

I used that page. Looking at it closer, I see that you are assembling the gauge on the gauge backing plate (can), not directly on the gauge. I have a spare center console gauge cluster and I'm going to dig it out. I'm thinking that I'm shorting out the gauge just like if on the leads short to the can as you pointed out. Now I know why I did not see that hole! At least I have a new avenue. Thanks Hugh

 

Found the can, cleaned it up and carefully install my new gauge. Nothing different!@#? See pictures. I guess I'm going to install a mechanical gauge and quit wasting my time and that of others. Thanks to all.




Cleaned can





No leads connected





The power lead and ground or ohm lead connected





All three leads connected.

 

I’m venting here.
I think I just achieved the title “dumbest electrician in the world”. I was attempting to resolve what I initially thought was a simple situation; my temperature gauge did not work. I’m a hobbyist/backyard mechanic that basically was introduced to the more involved auto mechanics working on various Vettes that I own.
Not only did I get some great info from the Forum, the responses were quick and informative. Wilcox details how to assemble a temperature gauge they provided pictures of testing various specific condition that might occur on a unit. Sadly for me they missed the one picture I needed, the one showing the needles position if the pigtail is installed backwards. The needle goes to the far left, just below the ‘100’ mark, the 9:30 position.
Please don’t ask how that could happen as we all laugh!! This Vette's interior was taken apart by me over 15 years ago. I had to remove the interior to get to a broken A/C distribution part and a damaged left side dash. Yes, I forgot how it can apart so I relied on my 1968 AIM. I colored the wire diagram and all. I faithfully refer to the appropriate sections as I rebuilt the car.
When asked by a friend if I installed the connector correctly, I again checked the wire diagram in the AIM electrical section and I did it exactly as it showed; RED lead on TOP, BLACK lead in the MIDDLE and GREEN lead on the BOTTOM. GUESS WHAT, THAT IS *** BACKWARDS!@#$%.
A will admit to my lack of knowledge about auto mechanics but I have years of experience in what should expected of a wiring diagram. When possible any illustration should reflect the parts actual function, ie, you don’t show a normally closed switch as a normally open switch. This diagram leaves a great deal to be desired.
Three gauges (one brand new), a new sending unit and a thread on the Forum later, I started looking into mechanical gauges!! I kept thinking to myself that I somehow reversed the polarity. As part of the search for the mechanical gauge and how electrical temperature gauges worked, I stumbled across a photo showing an electric temperature gauge with the lead colors painted on the gauge's lead. PROBLEM SOLVED.
Where do I pick up my award??